Devices for the automated release of liquid medicaments are normally used with patients who have a continuous or variable need of a medicine that can be administered by subcutaneous infusion. Specific applications are, for example, certain pain therapies and the treatment of diabetes, in which computer controlled infusion pump devices, such as insulin pumps, are used. Such devices can be carried by a patient on the body and contain a certain amount of liquid medicament in a reservoir in the form of a container. The medicine reservoir often comprises medicine sufficient for one or more days. The liquid medicament is supplied to the patient's body from the reservoir through an infusion cannula or an injection needle.
In self-administration of medicaments, such as the self administration of insulin, the patients administering the medicament by means of an infusion pump device are increasingly emphasizing convenience and discretion. As a consequence, such infusion devices are designed to be as small as possible to increase discretion and improve patient comfort.
While there are fully or partially disposable single-use infusion pump devices, such devices are typically non-disposable and are loaded with a disposable drug cartridge. Such disposable cartridges are preferable for sterility and contamination prevention reasons. They may be delivered pre-filled with a certain liquid medicament, or empty, ready to be filled by a user.
One common type of infusion pump device that is carried on or near the body has a medicine reservoir with a cylindrical ampoule and a displacement piston, which is pushed into the ampoule by a piston rod or threaded spindle in order to convey the liquid medicament. These known designs have the disadvantage of being longer and/or thicker than desired.
In another type of infusion pump device the rigid container and movable piston of a syringe-type infusion pump are replaced by a flexible container. Such a container may, for example, have the form of two flexible wall sheets that are sealed together. The liquid medicament is obtained from the container by a downstream pump. Flexible containers have the advantage of a smaller volume surplus of the container in relation to its content, which reduces manufacturing costs and the achievable dimensions of an infusion pump device using such a flexible container. The volume of a flexible container for use in an infusion pump device may be up to 10 ml, but is preferably 5 ml or less, and more preferably lies in a range of 1.5 to 3.5 ml.
A known problem of infusion devices are air bubbles in the fluidic system, particularly in the pump system, but also in other components such as the container or a filling port. A certain volume of air present in the container cannot be avoided or completely removed. If said air remains in the container or in another part of the fluidic system, it may be administered in place of the liquid medicament, which leads to potentially dangerous dosing errors. Furthermore, the administration of air into a patient's body should be generally avoided for medical reasons.
Yet another problem of air in the fluidic system is the reduced stiffness of the fluidic system. Due to the high compressibility of gases such as air in relation to liquids such as water, it becomes difficult to measure the exact pressure in the fluidic system. This impedes the detection of blockages or occlusions in the fluidic system by measuring the fluidic pressure.